1. Field of the invention:
The present invention relates to a television camera (hereinafter TV camera) for endoscopes.
2. Description of the prior art:
In the recent years, it is increasingly practiced to observe narrow locations such as interiors of coelomata in the form of images on TV screens by attaching cameras to eyepieces of fiberscopes. FIG. 1 shows a schematic sectional view illustrating a fiberscope to which a TV camera is attached. As shown in this drawing, a fiberscope 10 comprises an objective lens 1, an image guide fiber bundle 2 and an eyepiece lens 3, and an image of an object is generally observed through the eyepiece lens 3. A TV camera 20 attached to the eyepiece 3 of the fiberscope 10 comprises an imaging lens 5 and a solid-state image sensor 6, and an image of an object formed on the exit end surface of the image guide fiber bundle 2 is focused by the eyepiece lens 3 and the imaging lens 5 again onto the light receiving surface of the solid-state image sensor 6 for enabling observation of the image through the TV camera.
The exit end surface of the image guide fiber bundle has a mesh structure wherein a multiple number of optical fibers are regularly arranged. When an image of the end surface is projected onto the solid-state image sensor 6, the mesh structure is reproduced and, since the period of the mesh structure is close to the period of the arrangement of the picture elements in the image sensor or that of the arrangement of the filter elements of the color encoding filter array used in combination with the image sensor as occasion demands, false signals called "moire (or aliasing) are produced in the image formed on the TV screen. These TV cameras produce the false signals far more remarkably than the general TV cameras.
In order to eliminate the moireit is general to use optical low pass filters. U.S. Pat. No. 4,100,570, for example, uses an optical low pass filter consisting of a birefringent plate such as quartz arranged in the imaging optical path of a TV image pick up system for eliminating the moireFurther, U.S. Pat. No. 4,417,272 adopts an optical low pass filter consisting of a prism.
A TV camera applying these techniques is disclosed by U.S. Pat. No. 4,676,593. This TV camera is so adapted as to eliminate the moire by arranging an optical low pass filter such as a quartz plate or a phase filter with an adapter lens which is disposed in the eyepiece optical system or between the TV camera and the eyepiece lens at the stage to attach the TV camera to the eyepiece of an endoscope. However, it is impossible to completely eliminate the moire in a TV camera for endoscopes, separately from the general TV cameras, simply by arranging an optical low pass filter as in the case of the above-mentioned example. This is because thickness of image guide fiber bundles is various and thickness of optical fibers proper composing the optical fiber bundles is also various for individual endoscopes used, whereby periods of mesh structures in the images formed on the light receiving surfaces of image sensors are different even for the same TV camera. When the above-mentioned periods are different, the moire is produced in various manners and cannot be eliminated sufficiently with a single optical low pass filter.
Further, Japanese Unexamined Published Utility Model Appln. No. 12145/61 discloses a TV camera so adapted as to use selectively, in accordance with objects to be photographed, a plural number of optical low pass filters which are prepared in advance. However, this example is undesirable since it inevitably enlarges the TV camera and complicates the composition thereof.
As another means to eliminate the moireU.S. Pat. No. 4,720,637 discloses a means to impart a relatively large spherical aberration to the imaging lens itself and limit spatial frequency spectrum of an image formed on the light receiving surface of the image sensor to the Nyquist frequency of the image sensor by the spherical aberration. Though this correcting means is applicable to the ordinary TV cameras, it cannot sufficiently eliminate the moire which is very remarkable in the TV camera for endoscopes.